<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<style>
  .demo{
    width: 20px;
    height: 20px;
    border-radius: 50%;
    background-color: red;
  }
</style>
<body>
  <!-- [html] 怎样在页面上实现一个圆形的可点击区域？
   -->
   <div>
     <div class="demo"  onclick="alert(3)"></div>
   </div>

   <svg width="100%" height="100%" version="1.1"
xmlns="http://www.w3.org/2000/svg">

<circle cx="100" cy="50" r="40" stroke="black"
stroke-width="2" fill="red" onclick="alert(3)"/>

</svg>
</body>
</html>

